on closer inspection of the Modeling section of GW's site I came a cross both.

Fantasy/ Other
GW wrote:
Citadel Warhammer Basing Kit Price: $30.00
Make your important units stand out on the battlefield with the Citadel Warhammer Basing Kit.
This box set contains: two tubs of slate(medium and large bits), two tubs of resin details, one spare tub and one frame of brass leaves.

40K/ Futuristic
GW wrote:
Citadel Warhammer 40,000 Basing Kit Price: $30.00
Make your important units stand out on the battlefield with the Citadel Warhammer 40,000 Basing Kit.
This box set contains: two tubs of slate, two tubs of resin details, one spare tub and one frame of brass etching
I've been wanting to try adding things to the bases like larger rocks and what not for awhile.
I have a large tub of small multie sized stones and a small tub of fine stones and a small tub static grass(small tubs from the Hobby Starter Set.) and I like the effect the small multie sized stones have on the basses.
when it comes to converting a model I can come up with ideas till the cows come home, but sadly when it comes to the bases I'm a little lacking. the most I've done to a base was take a spru piece stand it vertical add two flat pieces of plastic on ether side to make an old style sign post. and I only did that cause I saw it online.... a GW article I think, and cause I think the clear plastic base & shaft for the Fell Bats is weak.